In order to understand gene activity and regulation in eucaryotes, it is necessary to know the details of the structure of chromatin, the DNA-protein complex upon which control elements must act. Experiments are proposed to investigate a number of aspects of chromatin structure using direct observation with the electron microscope as the primary tool. specific areas of research are: the reconstitution of nucleosomes and multinucleosomal pieces of chromatin from the separated components; the packaging of nucleosomes into higher order chromosomal structures; the structure of replicating chromatin; the development of immunological probes of chromatin structure; and the development and assessment of techniques for preparing chromatin for high resolution electron microscopy.